(106).Weather

A hurricane feeds off warm water. This NASA map of sea surface temperature shows just how warm the water was in the tropical Atlantic and the Gulf of Mexico in August 2005 when Hurricane Katrina made its way towards the Gulf Coast.

The storm began over the southeastern Bahamas on August 23 and moved over south Florida the next day as a Category 1 hurricane. The storm killed nine people and caused about $600 million in damage. As the storm traveled west over the Gulf of Mexico, the water was abnormally warm, as high as 89°F (32°C). On August 27, the storm was upgraded to Category 3 and the next day it received the highest designation, Category 5. Winds of 175 mph (280 kph) and gusts of 215 mph (344 kph) were reported. The residents of New Orleans were advised to evacuate the city and fortunately many did.

By the time Hurricane Katrina hit land it had been downgraded to a Category 4 storm. New Orleans was not hit head-on, but by the weaker side of the storm. Initial reports were that the city had been spared. What people didn’t know initially was that storm surge had collapsed several sections of the levee that protected the city. Soon 80% of the city was submerged; around 1,300 people were dead (2,500 throughout the region) and one million people were homeless.

^{Courtesy of NASA/SVS. www.nasa.gov/vision/earth/lookingatearth/h2005_katrina.html. Public Domain.}

(105).Air movement

Vocabulary

advection

Chinook winds (Foehn winds)

haboob

high pressure zone

jet stream

katabatic winds

land breeze

low pressure zone

monsoon

mountain breeze

polar front

rainshadow effect

Santa Ana winds

sea breeze

valley breeze

Introduction

A few basic principles go a long way toward explaining how and why air moves: Warm air rising creates a low pressure zone at the ground. Air from the surrounding area is sucked into the space left by the rising air. Air flows horizontally at top of the troposphere; horizontal flow is called advection. The air cools until it descends. Where it reaches the ground, it creates a high pressure zone. Air flowing from areas of high pressure to low pressure creates winds. Warm air can hold more moisture than cold air. Air moving at the bases of the three major convection cells in each hemisphere north and south of the equator creates the global wind belts.

Air Pressure and Winds

Within the troposphere are convection cells (Figure below).

[Figure 1]

Warm air rises, creating a low pressure zone; cool air sinks, creating a high pressure zone.

Air that moves horizontally between high and low pressure zones makes wind. The greater the pressure difference between the pressure zones the faster the wind moves.

Convection in the atmosphere creates the planet’s weather. When warm air rises and cools in a low pressure zone, it may not be able to hold all the water it contains as vapor. Some water vapor may condense to form clouds or precipitation. When cool air descends, it warms. Since it can then hold more moisture, the descending air will evaporate water on the ground.

Air moving between large high and low pressure systems creates the global wind belts that profoundly affect regional climate. Smaller pressure systems create localized winds that affect the weather and climate of a local area.

Local Winds

Local winds result from air moving between small low and high pressure systems. High and low pressure cells are created by a variety of conditions. Some local winds have very important effects on the weather and climate of some regions.

Land and Sea Breezes

Since water has a very high specific heat, it maintains its temperature well. So water heats and cools more slowly than land. If there is a large temperature difference between the surface of the sea (or a large lake) and the land next to it, high and low pressure regions form. This creates local winds.

Sea breezes blow from the cooler ocean over the warmer land in summer (Figure below). Where is the high pressure zone and where is the low pressure zone? Sea breezes blow at about 10 to 20 km (6 to 12 miles) per hour and lower air temperature much as 5 to 10°C (9 to 18°F).

Land breezes blow from the land to the sea in winter. Where is the high pressure zone and where is the low pressure zone? Some warmer air from the ocean rises and then sinks on land, causing the temperature over the land to become warmer.

[Figure 2]

How do sea and land breezes moderate coastal climates?

Land and sea breezes create the pleasant climate for which Southern California is known. The effect of land and sea breezes are felt only about 50 to 100 km (30 to 60 miles) inland. This same cooling and warming effect occurs to a smaller degree during day and night, because land warms and cools faster than the ocean.

Monsoon Winds

Monsoon winds are larger scale versions of land and sea breezes; they blow from the sea onto the land in summer and from the land onto the sea in winter. Monsoon winds are occur where very hot summer lands are next to the sea. Thunderstorms are common during monsoons (Figure below).

[Figure 3]

In the southwestern United States relatively cool moist air sucked in from the Gulf of Mexico and the Gulf of California meets air that has been heated by scorching desert temperatures.

The most important monsoon in the world occurs each year over the Indian subcontinent. More than two billion residents of India and southeastern Asia depend on monsoon rains for their drinking and irrigation water. Back in the days of sailing ships, seasonal shifts in the monsoon winds carried goods back and forth between India and Africa.

Mountain and Valley Breezes

Temperature differences between mountains and valleys create mountain and valley breezes. During the day, air on mountain slopes is heated more than air at the same elevation over an adjacent valley. As the day progresses, warm air rises and draws the cool air up from the valley, creating a valley breeze. At night the mountain slopes cool more quickly than the nearby valley, which causes a mountain breeze to flow downhill.

Katabatic Winds

Katabatic winds move up and down slopes, but they are stronger mountain and valley breezes. Katabatic winds form over a high land area, like a high plateau. The plateau is usually surrounded on almost all sides by mountains. In winter, the plateau grows cold. The air above the plateau grows cold and sinks down from the plateau through gaps in the mountains. Wind speeds depend on the difference in air pressure over the plateau and over the surroundings. Katabatic winds form over many continental areas. Extremely cold katabatic winds blow over Antarctica and Greenland.

Chinook Winds (Foehn Winds)

Chinook winds (or Foehn winds) develop when air is forced up over a mountain range. This takes place, for example, when the westerly winds bring air from the Pacific Ocean over the Sierra Nevada Mountains in California. As the relatively warm, moist air rises over the windward side of the mountains, it cools and contracts. If the air is humid, it may form clouds and drop rain or snow. When the air sinks on the leeward side of the mountains, it forms a high pressure zone. The windward side of a mountain range is the side that receives the wind; the leeward side is the side where air sinks.

The descending air warms and creates strong, dry winds. Chinook winds can raise temperatures more than 20°C (36°F) in an hour and they rapidly decrease humidity. Snow on the leeward side of the mountain disappears melts quickly. If precipitation falls as the air rises over the mountains, the air will be dry as it sinks on the leeward size. This dry, sinking air causes a rainshadow effect (Figurebelow), which creates many of the world’s deserts.

[Figure 4]

As air rises over a mountain it cools and loses moisture, then warms by compression on the leeward side. The resulting warm and dry winds are Chinook winds. The leeward side of the mountain experiences rainshadow effect.

Santa Ana Winds

Santa Ana winds are created in the late fall and winter when the Great Basin east of the Sierra Nevada cools, creating a high pressure zone. The high pressure forces winds downhill and in a clockwise direction (because of Coriolis). The air pressure rises, so temperature rises and humidity falls. The winds blow across the Southwestern deserts and then race downhill and westward toward the ocean. Air is forced through canyons cutting the San Gabriel and San Bernardino mountains (Figure below).

[Figure 5]

The winds are especially fast through Santa Ana Canyon, for which they are named. Santa Ana winds blow dust and smoke westward over the Pacific from Southern California.

The Santa Ana winds often arrive at the end of California’s long summer drought season. The hot, dry winds dry out the landscape even more. If a fire starts, it can spread quickly, causing large-scale devastation (Figure below).

[Figure 6]

In October 2007, Santa Ana winds fueled many fires that together burned 426,000 acres of wild land and more than 1,500 homes in Southern California.

Desert Winds

High summer temperatures on the desert create high winds, which are often associated with monsoon storms. Desert winds pick up dust because there is not as much vegetation to hold down the dirt and sand. (Figure below). A haboobforms in the downdrafts on the front of a thunderstorm.

[Figure 7]

A haboob in the Phoenix metropolitan area, Arizona.

Dust devils, also called whirlwinds, form as the ground becomes so hot that the air above it heats and rises. Air flows into the low pressure and begins to spin. Dust devils are small and short-lived but they may cause damage.

Atmospheric Circulation

Because more solar energy hits the equator, the air warms and forms a low pressure zone. At the top of the troposphere, half moves toward the North Pole and half toward the South Pole. As it moves along the top of the troposphere it cools. The cool air is dense and when it reaches a high pressure zone it sinks to the ground. The air is sucked back toward the low pressure at the equator. This describes the convection cells north and south of the equator.

If the Earth did not rotate, there would be one convection cell in the northern hemisphere and one in the southern with the rising air at the equator and the sinking air at each pole. But because the planet does rotate, the situation is more complicated. The planet’s rotation means that the Coriolis Effect must be taken into account. Coriolis Effect was described in the Earth's Oceans chapter.

Let’s look at atmospheric circulation in the Northern Hemisphere as a result of the Coriolis Effect (Figure below). Air rises at the equator, but as it moves toward the pole at the top of the troposphere, it deflects to the right. (Remember that it just appears to deflect to the right because the ground beneath it moves.) At about 30°N latitude, the air from the equator meets air flowing toward the equator from the higher latitudes. This air is cool because it has come from higher latitudes. Both batches of air descend, creating a high pressure zone. Once on the ground, the air returns to the equator. This convection cell is called the Hadley Cell and is found between 0° and 30°N.

[Figure 8]

The atmospheric circulation cells, showing direction of winds at Earth's surface.

There are two more convection cells in the Northern Hemisphere. The Ferrell cell is between 30°N and 50° to 60°N. This cell shares its southern, descending side with the Hadley cell to its south. Its northern rising limb is shared with the Polar cell located between 50°N to 60°N and the North Pole, where cold air descends.

There are three mirror image circulation cells in the Southern Hemisphere. In that hemisphere, the Coriolis Effect makes objects appear to deflect to the left.

Global Wind Belts

Global winds blow in belts encircling the planet. The global wind belts are enormous and the winds are relatively steady (Figure below). These winds are the result of air movement at the bottom of the major atmospheric circulation cells, where the air moves horizontally from high to low pressure.

[Figure 9]

The major wind belts and the directions that they blow.

Global Wind Belts

Let’s look at the global wind belts in the Northern Hemisphere.

In the Hadley cell air should move north to south, but it is deflected to the right by Coriolis. So the air blows from northeast to the southwest. This belt is the trade winds, so called because at the time of sailing ships they were good for trade.

In the Ferrel cell air should move south to north, but the winds actually blow from the southwest. This belt is the westerly winds or westerlies. Why do you think a flight across the United States from San Francisco to New York City takes less time than the reverse trip?

In the Polar cell, the winds travel from the northeast and are called the polar easterlies

The wind belts are named for the directions from which the winds come. The westerly winds, for example, blow from west to east. These names hold for the winds in the wind belts of the Southern Hemisphere as well.

This video lecture discusses the 3-cell model of atmospheric circulation and the resulting global wind belts and surface wind currents (5a): http://www.youtu

Global Winds and Precipitation

Besides their effect on the global wind belts, the high and low pressure areas created by the six atmospheric circulation cells determine in a general way the amount of precipitation a region receives. In low pressure regions, where air is rising, rain is common. In high pressure areas, the sinking air causes evaporation and the region is usually dry. More specific climate effects will be described in the chapter about climate.

Polar Fronts and Jet Streams

The polar front is the junction between the Ferrell and Polar cells. At this low pressure zone, relatively warm, moist air of the Ferrell Cell runs into relatively cold, dry air of the Polar cell. The weather where these two meet is extremely variable, typical of much of North America and Europe.

The polar jet stream is found high up in the atmosphere where the two cells come together. A jet stream is a fast-flowing river of air at the boundary between the troposphere and the stratosphere. Jet streams form where there is a large temperature difference between two air masses. This explains why the polar jet stream is the world’s most powerful (Figure below).

[Figure 10]

A cross section of the atmosphere with major circulation cells and jet streams. The polar jet stream is the site of extremely turbulent weather.

Jet streams move seasonally just as the angle of the Sun in the sky moves north and south. The polar jet stream, known as “the jet stream,” moves south in the winter and north in the summer between about 30°N and 50° to 75°N.

Lesson Summary

Winds blow from high pressure zones to low pressure zones. The pressure zones are created when air near the ground becomes warmer or colder than the air nearby.

Local winds may be found in a mountain valley or near a coast.

The global wind patterns are long-term, steady winds that prevail around a large portion of the planet.

The location of the global wind belts has a great deal of influence on the weather and climate of an area.

(104).Atmospheric layers.

Vocabulary

aurora

exosphere

inversion

ionosphere

magnetosphere

mesosphere

ozone layer

solar wind

stratosphere

temperature gradient

thermosphere

troposphere

Introduction

The atmosphere is layered, corresponding with how the atmosphere’s temperature changes with altitude. By understanding the way temperature changes with altitude, we can learn a lot about how the atmosphere works. While weather takes place in the lower atmosphere, interesting things, such as the beautiful aurora, happen higher in the atmosphere.

Air Temperature

[Figure 1]

Papers held up by rising air currents above a radiator demonstrate the important principle that warm air rises.

Why does warm air rise (Figure above)? Gas molecules are able to move freely and if they are uncontained, as they are in the atmosphere, they can take up more or less space.

When gas molecules are cool, they are sluggish and do not take up as much space. With the same number of molecules in less space, both air density and air pressure are higher.

When gas molecules are warm, they move vigorously and take up more space. Air density and air pressure are lower.

Warmer, lighter air is more buoyant than the cooler air above it, so it rises. The cooler air then sinks down, because it is denser than the air beneath it. This is convection, which was described in the Plate Tectonics chapter.

The property that changes most strikingly with altitude is air temperature. Unlike the change in pressure and density, which decrease with altitude, changes in air temperature are not regular. A change in temperature with distance is called a temperature gradient.

The atmosphere is divided into layers based on how the temperature in that layer changes with altitude, the layer’s temperature gradient (Figure below). The temperature gradient of each layer is different. In some layers, temperature increases with altitude and in others it decreases. The temperature gradient in each layer is determined by the heat source of the layer (Figure below).

[Figure 2]

The four main layers of the atmosphere have different temperature gradients, creating the thermal structure of the atmosphere.

[Figure 3]

The layers of the atmosphere appear as different colors in this image from the International Space Station.

Most of the important processes of the atmosphere take place in the lowest two layers: the troposphere and the stratosphere.

Troposphere

The temperature of the troposphere is highest near the surface of the Earth and decreases with altitude. On average, the temperature gradient of the troposphere is 6.5°C per 1,000 m (3.6°F per 1,000 ft.) of altitude. What is the source of heat for the troposphere?

Earth’s surface is a major source of heat for the troposphere, although nearly all of that heat comes from the Sun. Rock, soil, and water on Earth absorb the Sun’s light and radiate it back into the atmosphere as heat. The temperature is also higher near the surface because of the greater density of gases. The higher gravity causes the temperature to rise.

Notice that in the troposphere warmer air is beneath cooler air. What do you think the consequence of this is? This condition is unstable. The warm air near the surface rises and cool air higher in the troposphere sinks. So air in the troposphere does a lot of mixing. This mixing causes the temperature gradient to vary with time and place. The rising and sinking of air in the troposphere means that all of the planet’s weather takes place in the troposphere.

Sometimes there is a temperature inversion, air temperature in the troposphere increases with altitude and warm air sits over cold air. Inversions are very stable and may last for several days or even weeks. Inversions form:

Over land at night or in winter when the ground is cold. The cold ground cools the air that sits above it, making this low layer of air denser than the air above it.

Near the coast where cold seawater cools the air above it. When that denser air moves inland, it slides beneath the warmer air over the land.

Since temperature inversions are stable, they often trap pollutants and produce unhealthy air conditions in cities (Figurebelow).

[Figure 4]

Smoke makes a temperature inversion visible. The smoke is trapped in cold dense air that lies beneath a cap of warmer air.

At the top of the troposphere is a thin layer in which the temperature does not change with height. This means that the cooler, denser air of the troposphere is trapped beneath the warmer, less dense air of the stratosphere. Air from the troposphere and stratosphere rarely mix.

A science experiment that clearly shows how a temperature inversion traps air, along with whatever pollutants are in it, near the ground is seen in this video (5c): http://www.youtube.com/watch?v=LPvn9qhVFbM (2:50).

Stratosphere

Ash and gas from a large volcanic eruption may burst into the stratosphere, the layer above the troposphere. Once in the stratosphere, it remains suspended there for many years because there is so little mixing between the two layers. Pilots like to fly in the lower portions of the stratosphere because there is little air turbulence.

In the stratosphere, temperature increases with altitude. What is the heat source for the stratosphere? The direct heat source for the stratosphere is the Sun. Air in the stratosphere is stable because warmer, less dense air sits over cooler, denser air. As a result, there is little mixing of air within the layer.

The ozone layer is found within the stratosphere between 15 to 30 km (9 to 19 miles) altitude. The thickness of the ozone layer varies by the season and also by latitude.

The ozone layer is extremely important because ozone gas in the stratosphere absorbs most of the Sun’s harmful ultraviolet (UV) radiation. Because of this, the ozone layer protects life on Earth. High-energy UV light penetrates cells and damages DNA, leading to cell death (which we know as a bad sunburn). Organisms on Earth are not adapted to heavy UV exposure, which kills or damages them. Without the ozone layer to reflect UVC and UVB radiation, most complex life on Earth would not survive long (Figure below).

[Figure 5]

Even with the ozone layer, UVB radiation still manages to reach Earth's surface, especially where solar radiation is high.

Mesosphere

Temperatures in the mesospheredecrease with altitude. Because there are few gas molecules in the mesosphere to absorb the Sun’s radiation, the heat source is the stratosphere below. The mesosphere is extremely cold, especially at its top, about -90°C (-130°F).

The air in the mesosphere has extremely low density: 99.9% of the mass of the atmosphere is below the mesosphere. As a result, air pressure is very low (Figurebelow). A person traveling through the mesosphere would experience severe burns from ultraviolet light since the ozone layer which provides UV protection is in the stratosphere below. There would be almost no oxygen for breathing. Stranger yet, an unprotected traveler’s blood would boil at normal body temperature because the pressure is so low.

[Figure 6]

Meteors burn in the mesosphere even though the gas is very thin; these burning meteors are shooting stars.

Thermosphere and Beyond

[Figure 7]

The International Space Station (ISS) orbits within the upper part of the thermosphere, at about 320 to 380 km above the Earth.

The density of molecules is so low in the thermosphere that one gas molecule can go about 1 km before it collides with another molecule. Since so little energy is transferred, the air feels very cold (Figure above).

Within the thermosphere is the ionosphere. The ionosphere gets its name from the solar radiation that ionizes gas molecules to create a positively charged ion and one or more negatively charged electrons. The freed electrons travel within the ionosphere as electric currents. Because of the free ions, the ionosphere has many interesting characteristics.

At night, radio waves bounce off the ionosphere and back to Earth. This is why you can often pick up an AM radio station far from its source at night.

The Van Allen radiation belts are two doughnut-shaped zones of highly charged particles that are located beyond the atmosphere in the magnetosphere. The particles originate in solar flares and fly to Earth on the solar wind. Once trapped by Earth’s magnetic field, they follow along the field’s magnetic lines of force. These lines extend from above the equator to the North Pole and also to the South Pole then return to the equator.

When massive solar storms cause the Van Allen belts to become overloaded with particles, the result is the most spectacular feature of the ionosphere -- the nighttime aurora (Figure below). The particles spiral along magnetic field lines toward the poles. The charged particles energize oxygen and nitrogen gas molecules, causing them to light up. Each gas emits a particular color of light.

[Figure 8]

(a) Spectacular light displays are visible as the aurora borealis or northern lights in the Northern Hemisphere. (b) The aurora australis or southern lights encircles Antarctica.

There is no real outer limit to the exosphere, the outermost layer of the atmosphere; the gas molecules finally become so scarce that at some point there are no more. Beyond the atmosphere is the solar wind. The solar wind is made of high-speed particles, mostly protons and electrons, traveling rapidly outward from the Sun.

This video is very thorough in its discussion of the layers of the atmosphere. Remember that the chemical composition of each layer is nearly the same except for the ozone layer that is found in the stratosphere (8a): http://www.youtube.com/watch?v=S-YAKZoy1A0 (6:44).

KQED: Illuminating the Northern Lights

What would Earth's magnetic field look like if it were painted in colors? It would look like the aurora! This QUEST video looks at the aurora, which provides clues about the solar wind, Earth's magnetic field and Earth's atmosphere. Learn more in the video below: 

Lesson Summary

Features of the atmosphere change with altitude: density decreases, air pressure decreases, temperature changes vary.

Different temperature gradients create different layers within the atmosphere.

The lowest layer is the troposphere where most of the atmospheric gases and all of the planet’s weather are located. The troposphere is heated from the ground, so temperature decreases with altitude. Because warm air rises and cool air sinks, the troposphere is unstable.

In the stratosphere, temperature increases with altitude. The stratosphere contains the ozone layer, which protects the planet from the Sun’s harmful UV radiation.

(103).The atmosphere

Vocabulary

air pressure

altitude

atmosphere

greenhouse gas

humidity

ozone

respiration

ultraviolet (UV) radiation

water vapor

weather

Introduction

Earth’s atmosphere is a thin blanket of gases and tiny particles — together called air. We are most aware of air when it moves and creates wind. All living things need some of the gases in air for life support. Without an atmosphere, Earth would likely be just another lifeless rock.

Significance of the Atmosphere

Earth's atmosphere, along with the abundant liquid water at Earth's surface, are the keys to our planet's unique place in the solar system. Much of what makes Earth exceptional depends on the atmosphere. Let's consider some of the reasons we are lucky to have an atmosphere.

Atmospheric Gases Are Indispensable for Life on Earth

Without the atmosphere, Earth would look a lot more like the Moon. Atmospheric gases, especially carbon dioxide (CO₂) and oxygen (O₂), are extremely important for living organisms. How does the atmosphere make life possible? How does life alter the atmosphere?

In photosynthesis plants use CO₂ and create O₂. Photosynthesis is responsible for nearly all of the oxygen currently found in the atmosphere. The chemical reaction for photosynthesis is:

6CO₂ + 6H₂O + solar energy → C₆H₁₂O₆(sugar) + 6O₂

By creating oxygen and food, plants have made an environment that is favorable for animals. In respiration, animals use oxygen to convert sugar into food energy they can use. Plants also go through respiration and consume some of the sugars they produce.

The chemical reaction for respiration is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + useable energy

How is respiration similar to and different from photosynthesis? They are approximately the reverse of each other. In photosynthesis, CO₂ is converted to O₂ and in respiration, O₂ is converted to CO₂ (Figure below).

[Figure 1]

Chlorophyll indicates the presence of photosynthesizing plants as does the vegetation index.

The Atmosphere is a Crucial Part of the Water Cycle

As part of the hydrologic cycle, which was detailed in the Earth's Fresh Water chapter, water spends a lot of time in the atmosphere, mostly as water vapor.

All weather takes place in the atmosphere, virtually all of it in the lower atmosphere. Weather describes what the atmosphere is like at a specific time and place, and may include temperature, wind, and precipitation. Weather is the change we experience from day to day. Climate is the long-term average of weather in a particular spot. Although the weather for a particular winter day in Tucson, Arizona, may include snow, the climate of Tucson is generally warm and dry.

Ozone in the Upper Atmosphere Makes Life on Earth Possible

Ozone is a molecule composed of three oxygen atoms, (O₃). Ozone in the upper atmosphere absorbs high-energy ultraviolet (UV) radiation coming from the Sun. This protects living things on Earth’s surface from the Sun’s most harmful rays. Without ozone for protection, only the simplest life forms would be able to live on Earth.

The Atmosphere Keeps Earth’s Temperature Moderate

Along with the oceans, the atmospherekeeps Earth's temperatures within an acceptable range. Greenhouse gasestrap heat in the atmosphere so they help to moderate global temperatures (Figurebelow). Without an atmosphere with greenhouse gases, Earth's temperatures would be frigid at night and scorching during the day. Important greenhouse gases include carbon dioxide, methane, water vapor, and ozone.

[Figure 2]

Fires, such as these set to burn forests across southeast Asia, contribute greenhouse gases to the atmosphere.

Atmospheric Gases Provide the Substance for Waves to Travel Through

The atmosphere is made of gases that take up space and transmit energy. Sound waves are among the types of energy that travel though the atmosphere. Without an atmosphere, we could not hear a single sound. Earth would be as silent as outer space. Of course, no insect, bird, or airplane would be able to fly because there would be no atmosphere to hold it up. Explosions in movies about space should be silent.

Composition of Air

Nitrogen and oxygen together make up 99% of the planet’s atmosphere. The rest of the gases are minor components but sometimes are very important (Figurebelow).

[Figure 3]

Nitrogen and oxygen make up 99% of the atmosphere; carbon dioxide is a very important minor component.

Humidity is the amount of water vapor in the air. Humidity varies from place to place and season to season. This fact is obvious if you compare a summer day in Atlanta, Georgia, where humidity is high, with a winter day in Phoenix, Arizona, where humidity is low. When the air is very humid, it feels heavy or sticky. Dry air usually feels more comfortable.

Where around the globe is mean atmospheric water vapor higher and where is it lower and why (Figurebelow)? Higher humidity is found around the equatorial regions because air temperatures are higher and warm air can hold more moisture than cooler air. Of course, humidity is lower near the polar regions because air temperature is lower.

[Figure 4]

Mean winter atmospheric water vapor in the Northern Hemisphere when temperature and humidity are lower than they would be in summer.

Some of what is in the atmosphere is not gas. Particles of dust, soil, fecal matter, metals, salt, smoke, ash, and other solids make up a small percentage of the atmosphere. Particles provide starting points (or nuclei) for water vapor to condense on and form raindrops. Some particles are pollutants, which are discussed in the Human Actions and the Atmosphere chapter.

Pressure and Density

The atmosphere has different properties at different elevations above sea level, or altitudes. The air density (the number of molecules in a given volume) decreases with increasing altitude. This is why people who climb tall mountains, such as Mt. Everest, have to set up camp at different elevations to let their bodies get used to the decreased air (Figure below).

Why does air density decrease with altitude? Gravity pulls the gas molecules towards Earth’s center. The pull of gravity is stronger closer to the center at sea level. Air is denser at sea level where the gravitational pull is greater.

Gases at sea level are also compressed by the weight of the atmosphere above them. The force of the air weighing down over a unit of area is known as its atmospheric pressure, or air pressure. Why are we not crushed? The molecules inside our bodies are pushing outward to compensate. Air pressure is felt from all directions, not just from above.

[Figure 5]

This bottle was closed at an altitude of 3,000 meters where air pressure is lower. When it was brought down to sea level, the higher air pressure caused the bottle to collapse.

At higher altitudes the atmospheric pressure is lower and the air is less dense than at higher altitudes. If your ears have ever "popped", you have experienced a change in air pressure. Gas molecules are found inside and outside your ears. When you change altitude quickly, like when an airplane is descending, your inner ear keeps the density of molecules at the original altitude. Eventually the air molecules inside your ear suddenly move through a small tube in your ear to equalize the pressure. This sudden rush of air is felt as a popping sensation.

Although the density of the atmosphere changes with altitude, the composition stays the same with altitude, with one exception. In the ozone layer, at about 20 km to 40 km above the surface, there is a greater concentration of ozone molecules than in other portions of the atmosphere

Lesson Summary

Without its atmosphere, Earth would be a very different planet. Gases in the atmosphere allow plants to photosynthesize and animals and plants to engage in respiration.

Water vapor, which is an atmospheric gas, is an essential part of the water cycle.

Although the amount of gases do not vary relative to each other in the atmosphere, there is one exception: the ozone layer. Ozone in the upper atmosphere protects life from the Sun’s high energy ultraviolet radiation.

Air pressure varies with altitude and temperature.

(102).Earth's atmospherw

Astronauts took this photo of the Moon barely visible above Earth’s atmosphere. Earth’s blue halo appears because the atmosphere scatters blue light more than other wavelengths. At the top of the atmosphere, gases become so thin that they just cease to exist and then there is nothing but empty space. Since there is no easy way to define the top of the atmosphere, scientists say that it is 100 km above Earth’s surface. At that location, solar energy enters the Earth system mostly as visible light. Energy as reflected light and heat leave the Earth system there. If average global temperature remains the same, the incoming and outgoing energy are equal. If more energy is coming in than going out, global temperatures increase. If more energy is going out than coming in, global temperatures decrease. Increases or decreases in greenhouse gases can change this energy balance. Clouds appear in Earth’s atmosphere where there is water vapor. Clouds, along with snow and ice, reflect sunlight and play an important role in global climate. Where clouds reflect light back into space, they reduce the energy in the atmosphere. But water vapor is a greenhouse gas, so clouds can also trap heat. Scientists are interested in the effects of clouds on Earth’s heat balance.

^{Courtesy of NASA's Earth Observatory. earthobservatory.nasa.gov/IOTD/view.php?id=7373. Public Domain.}

(101).Ocean life.

Vocabulary

chemosynthesis

hydrothermal vent

invertebrate

phytoplankton

plankton

primary productivity

reef

vertebrate

zooplankton

Introduction

Oceans are a harsh placed to live. In the intertidal zone, conditions change rapidly as water covers and uncovers the region and waves pound on the rocks. Most of the environments at sea are cold and at just about any depth below the surface the pressure is very high. Beyond the photic zone, the ocean is entirely black. Organisms have adapted to these conditions in many interesting and effective ways. The size and variety of different habitats means that the oceans are home to a large portion of all life on Earth.

Types of Ocean Organisms

The smallest and largest animals on Earth live in the oceans. Why do you think the oceans can support large animals?

Marine animals breathe air or extract oxygen from the water. Some float on the surface and others dive into the ocean’s depths. There are animals that eat other animals, and plants generate food from sunlight. A few bizarre creatures break down chemicals to make food! The following section divides ocean life into seven basic groups.

Plankton

Plankton are organisms that cannot swim but that float along with the current. The word "plankton" comes from the Greek for wanderer. Most plankton are microscopic, but some are visible to the naked eye (Figure below).

Phytoplankton are tiny plants that make food by photosynthesis. Because they need sunlight, phytoplankton live in the photic zone. Phytoplankton are responsible for about half of the total primary productivity (food energy) on Earth. Like other plants, phytoplankton release oxygen as a waste product.

[Figure 1]

Microscopic diatoms are a type of phytoplankton.

A video of a research vessel sampling plankton is seen here: http://www.youtube.com/watch?v=mQG4zAoh6xc.

Zooplankton, or animal plankton, eat phytoplankton as their source of food (Figure below). Some zooplankton live as plankton all their lives and others are juvenile forms of animals that will attach to the bottom as adults. Some small invertebrates live as zooplankton.

[Figure 2]

Copepods are abundant and so are an important food source for larger animals.

Plants and Algae

The few true plants found in the oceans include salt marsh grasses and mangrove trees. Although they are not true plants, large algae, which are called seaweed, also use photosynthesis to make food. Plants and seaweeds are found in the neritic zone where the light they need penetrates so that they can photosynthesize (Figure below).

[Figure 3]

Kelp grow in forests in the neritic zone. Otters and other organisms depend on the kelp-forest ecosystem.

Marine Invertebrates

The variety and number of invertebrates, animals without a backbone, is truly remarkable (Figure below). Marine invertebrates include sea slugs, sea anemones, starfish, octopi, clams, sponges, sea worms, crabs, and lobsters. Most of these animals are found close to the shore, but they can be found throughout the ocean.

[Figure 4]

(a) Mussels; (b) Crown of thorns sea star; (c) Moon jelly; (d) A squid.

KQED: Amazing Jellies

Jellies are otherworldly creatures that glow in the dark, without brains or bones, some more than 100 feet long. Along with many other ocean areas, they live just off California's coast. Learn more in the video below: 

Fish

Fish are vertebrates; they have a backbone. What are some of the features fish have that allows them to live in the oceans? All fish have most or all of these traits.

Fins with which to move and steer.

Scales for protection.

Gills for extracting oxygen from the water.

A swim bladder that lets them rise and sink to different depths.

Ectothermy (cold-bloodedness) so that their bodies are the same temperature as the surrounding water.

Bioluminescence: light created from a chemical reaction that can attract prey or mates in the dark ocean.

Included among the fish are sardines, salmon, and eels, as well as the sharks and rays (which lack swim bladders) (Figure below).

[Figure 5]

The Great White Shark is a fish that preys on other fish and marine mammals.

Reptiles

Only a few types of reptiles live in the oceans and they live in warm water. Why are reptiles so restricted in their ability to live in the sea? Sea turtles, sea snakes, saltwater crocodiles, and marine iguana that are found only at the Galapagos Islands sum up the marine reptile groups (Figure below). Sea snakes bear live young in the ocean, but turtles, crocodiles, and marine iguanas all lay their eggs on land.

[Figure 6]

Sea turtles are found all over the oceans, but their numbers are diminishing.

Seabirds

Many types of birds are adapted to living in the sea or on the shore. A few are shown: (Figure below).

[Figure 7]

(a) With their long legs for wading and long bills for digging in sand for food, shorebirds are well adapted for the intertidal. (b) Many seabirds live on land but go to sea to fish, such as gulls, pelicans, and this frigate bird. (c) Albatross spend months at sea and only come on shore to raise chicks.

Marine Mammals

What are the common traits of mammals? Mammals are endothermic (warm-blooded) vertebrates that give birth to live young; feed them with milk; and have hair, ears, and a jaw bone with teeth.

What traits might mammals have to be adapted to life in the ocean?

For swimming: streamlined bodies, slippery skin or hair, fins.

For warmth: Fur, fat, high metabolic rate, small surface area to volume, specialized blood system.

For salinity: kidneys that excrete salt, impervious skin.

The five types of marine mammals are pictured here: (Figure below).

[Figure 8]

(a) Cetaceans: whales, dolphins, and porpoises. (b) Sirenians: manatee and the dugong. (c) Mustelids: Sea otters (terrestrial members are skunks, badgers and weasels). (d) Pinnipeds: Seals, sea lions, and walruses. (e) Polar bear.

KQED: Into the Deep with Elephant Seals

Thousands of northern elephant seals — some weighing up to 4,500 pounds — make an annual migration to breed each winter at Año Nuevo State Reserve in California. Marine biologists are using high-tech tools to explore the secrets of these amazing creatures. Learn more in the video below: 

Interactions Among Ocean Organisms

The previous section briefly discussed the adaptations different types of organisms have to live in the ocean. A look at a few of the different habitats organisms live in can focus even more on these important adaptations.

The Intertidal

A great abundance of life is found in the intertidal zone (Figure below). High energy waves pound the organisms that live in this zone and so they must be adapted to pounding waves and exposure to air during low tides. Hard shells protect from pounding waves and also protect against drying out when the animal is above water. Strong attachments keep the animals anchored to the rock.

[Figure 9]

In a tide pool, animals cling to the rock at low tide.

In a tide pool, as in the photo, what organisms are found where and what specific adaptations do they have to that zone? The mussels on the top left have hard shells for protection and to prevent drying because they are often not covered by water. The sea anemones in the lower right are more often submerged and have strong attachments but can close during low tides.

Many young organisms get their start in estuaries and so they must be adapted to rapid shifts in salinity.

Reefs

Corals and other animals deposit calcium carbonate to create rock reefs near the shore. Coral reefs are the “rainforests of the oceans” with a tremendous amount of species diversity (Figure below).

[Figure 10]

Coral reefs are among the most densely inhabited and diverse areas on the globe.

Reefs can form interesting shapes in the oceans. Remember that hot spots create volcanoes on the seafloor. If these volcanoes rise above sea level to become islands, and if they occur in tropical waters, coral reefs will form on them. Since the volcanoes are cones, the reef forms in a circle around the volcano. As the volcano comes off the hot spot, the crust cools. The volcano subsides and then begins to erode away (Figurebelow).

[Figure 11]

In this image of Maupiti Island in the South Pacific, the remnants of the volcano are surrounded by the circular reef.

Eventually, all that is left is a reef island called an atoll. A lagoon is found inside the reef (Figure below).

[Figure 12]

The Tuamotos are coral atolls that rest on volcanoes that are not beneath sea level.

Coral reef are near shore and so are subject to pollution from land. The coral animals are very sensitive to temperature and reefs around the world are stressed from rising ocean temperatures.

Some videos about threats to coral reefs are found at: National Geographic Videos, Environment Video, Threats to Animals, http://video.nationalgeographic.com/video/player/environment/.

Coral Reefs

Belize’s Coral Reef

Oceanic Zone

The open ocean is a vast area. Food either washes down from the land or is created by photosynthesizing plankton. Zooplankton and larger animals feed on the phytoplankton and on each other. Larger animals such as whales and giant groupers may live their entire lives in the open water.

How do fish survive in the deepest ocean? The few species that live in the greatest depths are very specialized (Figure below). Since it’s rare to find a meal, the fish use very little energy; they move very little, breathe slowly, have minimal bone structure and a slow metabolism. These fish are very small. To maximize the chance of getting a meal, some species may have jaws that unhinge to accept a larger fish or backward-folding teeth to keep prey from escaping.

[Figure 13]

An 1896 drawing of a deep sea angler fish with a bioluminescent “lure” to attract prey.

How we can know what lives in the ocean is in “Deep-Sea Robo Help”

Some of the results of the Census of Marine Life have been released and are discussed in “Record-Breaking Sea-Creature Surveys Released”

Bioluminescence is common in the oceans and seen in “Why Deep Sea Creatures Glow”

Hydrothermal Vents

At mid-ocean ridges at hydrothermal vents, bacteria that use chemosynthesisfor food energy are the base of a unique ecosystem (Figure below). This ecosystem is entirely separate from the photosynthesis at the surface. Shrimp, clams, fish, and giant tube worms have been found in these extreme places.

[Figure 14]

Giant tube worms found at hydrothermal vents get food from the chemosynthetic bacteria that live within them. The bacteria provide food; the worms provide shelter.

A video explaining hydrothermal vents with good footage is seen here: http://www.youtube.com/watch?v=rFHtVRKoaUM.

Lesson Summaries

The oceans have a tremendous diversity of life: bacteria, plankton, invertebrates, and vertebrates, which include fish, reptiles, seabirds, and mammals.

Photosynthesis and chemosynthesis create food energy in two very different ways.

Plankton are tiny freely floating plants (phytoplankton) or animals (zooplankton).

All marine organisms must be specialized for the harsh conditions of the ocean environment in which they live.

(100).The seafloor

Vocabulary

bottom trawling

manganese nodule

Introduction

Oceanographers like to say that we know more about the dark side of the Moon than we do about the oceans. That statement is doubly true of the seafloor. Although modern technology has allowed us to learn more about the seafloor, vast regions remain unexplored.

Studying the Seafloor

Scuba divers can only dive to about 40 meters and they cannot stay down there for very long. Although this is good for researching the organisms and ecosystems very near a coast, most oceanic research requires accessing greater depths.

Mapping

In the Plate Tectonics chapter you learned that echo sounders designed to locate enemy submarines allowed scientists to create bathymetric maps of the seafloor (Figure below). Prior to this advance, explorers mapped a small amount of the seafloor by painstakingly dropping a line over the side of a ship to measure the depth at one tiny spot at a time.

[Figure 1]

Map of San Francisco Bay, oblique view.

KQED: Sea 3-D: Charting the Ocean Floor

Using sound and laser technology, researchers have begun to reveal the secrets of the ocean floor from the Sonoma Coast to Monterey Bay. By creating complex 3-D maps, they're hoping to learn more about waves and achieve ambitious conservation goals. Learn more in the video below: 

Sampling Remotely

Samples of seawater from different depths in the water column are needed to understand ocean chemistry. To do this bottles are placed along a cable at regular depths and closed as a weight is dropped down the cable. The water trapped in the bottle can be analyzed later in a laboratory (Figure below).

[Figure 2]

A Niskin bottle being deployed off the side of a research ship.

Scientists are also interested in collecting rock and sediment samples from the seafloor. A dredge is a giant rectangular bucket that is dragged along behind a ship to collect loose rocks. Gravity corers are metal tubes that fall to the seafloor and slice into the sediments to collect a sample. The research vessel, the Joides Resolution, drills deep into the seafloor to collect samples of the sediment and ocean crust. Scientists analyze the samples for chemistry and paleomagnetism.

Submersibles

Samples of seawater and rocks can be collected directly by scientists in a submersible. These subs can take scientists down to make observations and the subs have arms for collecting samples. The submersible Alvin is an HOV, a human operated vehicle. Alvin can dive up to 4,500 m beneath the ocean surface and has made more than 4000 dives since 1964 (Figure below). Other submersibles can dive deeper.

[Figure 3]

Alvin allows two people and a pilot to make a nine hour dive.

Remotely Operated Vehicles

To avoid the expense, dangers, and limitations of human missions under the sea, remotely operated vehicles or ROV’s, allow scientists to study the ocean’s depths by using small vehicles carrying cameras and scientific instruments. ROVs were used to study the Titanic, which would have been far too dangerous for a manned sub to enter. Scientists control ROVs electronically with sophisticated operating systems (Figure below)

[Figure 4]

Remotely operated vehicles such as this one allow scientists to study the seafloor.

A video of the ROV Nereus from the Woods Hole Oceanographic Institution is shown here: http://www.youtube.com/watch?v=wwdF_2wMRfU.

Ocean Resources

The ocean provides important living and non-living resources. To be maintained for future use, these resources must be managed sustainably.

Living Resources

Most fish are caught by lines or nets as they swim in the open waters of the ocean. Some species of fish are being overharvested, which means their rate of reproduction cannot keep up with the rate at which people consume them.

Bottom trawling is a method of fishing that involves towing a weighted net across the seafloor to harvest fish. In many areas where bottom trawling is done, ecosystems are severely disturbed by the large nets. For this reason, in a few areas in the world, laws limit bottom trawling to waters not more than 1,000 m deep or waters far from protected and sensitive areas. Still these actions protect some of the seafloor. Besides food, ocean organisms have other uses. Some provide us with medications.

Non-living Resources

Oil and natural gas are the most valuable non-living resources taken from the ocean. Extracting these resources requires drilling into the seafloor. Oil platforms have dozens of oil wells that are drilled in places where the ocean is sometimes 2,000 m deep (Figure below). A description of the Deepwater Horizon oil spill affecting the Gulf of Mexico is located in the Human Actions and Earth's Waters chapter.

[Figure 5]

Oil platforms can be fixed or they can float.

The seafloor has some valuable minerals. Manganese nodules containing manganese, iron, copper, nickel, phosphate, and cobalt (Figure below) may be as small as a pea or as large as a basketball. Estimates are that there may be as much as 500 billion tons of nodules on the seafloor. The minerals in manganese nodules have many uses in the industrial world, but currently they are not being mined. Think back to the discussion of ore deposits in the Earth's Minerals chapter. Why do you think these seafloor resources are not being mined?

[Figure 6]

Manganese nodules from the seafloor are often rich in metals such as manganese, iron, nickel, copper, and cobalt.

Lesson Summary

Scuba divers can only explore near the surface so most oceanographic research is done from satellites, ships, and submersibles.

The oceans are divided into zones by water depth, distance from shore, and the slope of the seafloor.

The oceans provide us with both living and non-living resources.

Living oceanic resources include fish and invertebrates used for food.

The most valuable non-living resources from the oceans are oil and natural gas.

(99).Ocean movements.

Vocabulary

Coriolis effect

downwelling

gyre

high tide

longshore current

low tide

neap tide

rip current

spring tide

storm surge

surface current

thermohaline circulation

tidal range

tide

upwelling

wave

Introduction

Ocean water is constantly in motion: north-south, east-west, alongshore, and vertically. Seawater motions are the result of waves, tides, and currents (Figure below). Ocean movements are the consequence of many separate factors: wind, tides, Coriolis effect, water density differences, and the shape of the ocean basins. Water movements and their causes will be discussed in this lesson.

[Figure 1]

Ocean waves transfer energy through the water over great distances.

Waves

Waves have been discussed in previous chapters in several contexts: seismic waves traveling through the planet, sound waves traveling through seawater, and ocean waves eroding beaches. Waves transfer energy and the size of a wave and the distance it travels depends on the amount of energy that it carries.

Wind Waves

This lesson studies the most familiar waves, those on the ocean's surface. Ocean waves originate from wind blowing – steady winds or high storm winds - over the water. Sometimes these winds are far from where the ocean waves are seen. What factors create the largest ocean waves?

The largest wind waves form when the wind

is very strong

blows steadily for a long time

blows over a long distance

The wind could be strong, but if it gusts for just a short time, large waves won’t form.

Wind blowing across the water transfers energy to that water. The energy first creates tiny ripples that create an uneven surface for the wind to catch so that it may create larger waves. These waves travel across the ocean out of the area where the wind is blowing.

Remember that a wave is a transfer of energy. Do you think the same molecules of water that starts out in a wave in the middle of the ocean later arrive at the shore?

Water molecules in waves make circles or ellipses (Figure below). Energy transfers between molecules but the molecules themselves mostly bob up and down in place

[Figure 2]

The circles show the motion of a water molecule in a wind wave. Wave energy is greatest at the surface and decreases with depth. A shows that a water molecule travels in a circular motion in deep water. B shows that molecules in shallow water travel in an elliptical path because of the ocean bottom.

When does a wave break? Do waves only break when they reach shore? Waves break when they become too tall to be supported by their base. This can happen at sea but happens predictably as a wave moves up a shore. The energy at the bottom of the wave is lost by friction with the ground so that the bottom of the wave slows down but the top of the wave continues at the same speed. The crest falls over and crashes down.

Some of the damage done by storms is from storm surge. Water piles up at a shoreline as storm winds push waves into the coast. Storm surge may raise sea level as much as 7.5 m (25 ft), which can be devastating in a shallow land area when winds, waves, and rain are intense.

A wild video of “Storm Surge” can be seen on National Geographic Videos, Environment Video, Natural Disasters, Landslides, 

KQED: Science of Big Waves

Maverick waves are massive. Learning how they are generated can tell scientists a great deal about how the ocean creates waves and especially large waves. Learn more in the video below: 

 

 

 

 

Tsunami

Tsunami are described in the Earthquakes chapter as damaging waves that result from the sharp jolt to the water from an undersea earthquake. Landslides, meteorite impacts, or any other jolt to ocean water may form a tsunami (Figure below). Tsunami can travel at speeds of 800 kilometers per hour (500 miles per hour).

Tsunami have small wave heights and long wavelengths so they are usually unnoticed at sea. As the wave rides up the continental shelf the wave height increases.

A video explanation of tsunami is here: http://www.youtube.com/watch?v=StdqGoezNrY

The wave speed of a tsunami is also slowed by friction with the shallower ocean floor, which causes the wavelength to decrease, creating a much taller wave.

Many people caught in a tsunami have no warning of its approach. Since the wavelength is long, a long time can pass between crests or troughs onshore. In 1755 in Lisbon, an offshore earthquake caused a great deal of damage on land. People rushed to the open space of the shore and discovered that the water was flowing seaward fast. The trough of the tsunami wave reached shore first. People who went out onto the open beach were drowned when the crest of the wave reached shore.

[Figure 3]

A wave from the 2004 Boxing Day Tsunami hits the Maldives in the Indian Ocean.

KQED: Science on the SPOT: Watching the Tides

Large tsunami in the Indian Ocean and more recently Japan have killed hundreds of thousands of people in recent years. The west coast is vulnerable to tsunami since it sits on the Pacific Ring of Fire. Scientists are trying to learn everything they can about predicting tsunamis before a massive one strikes a little closer to home. Learn more in the video below: 

 

 

 

 

Tides

Tides are the daily rise and fall of sea level at any given place. The pull of the Moon’s gravity on Earth is the primarily cause of tides and the pull of the Sun’s gravity on Earth is the secondary cause (Figure below). The Moon has a greater effect because, although it is much smaller than the Sun, it is much closer. The Moon’s pull is about twice that of the Sun’s.

[Figure 4]

High tide (left) and low tide (right) at Bay of Fundy on the Gulf of Maine. The Bay of Fundy has the greatest tidal ranges on Earth at 38.4 feet.

Daily Tide Patterns

To understand the tides it is easiest to start with the effect of the Moon on Earth. As the Moon revolves around our planet, its gravity pulls Earth toward it. The lithosphere is unable to move much but the water is pulled by the gravity and a bulge is created. This bulge is the high tide beneath the Moon. The Moon's gravity then pulls the Earth toward it, leaving the water on the opposite side of the planet behind. This creates a second high tide bulge on the opposite side of Earth from the Moon. These two water bulges on opposite sides of the Earth aligned with the Moon are the high tides.

Since so much water is pulled into the two high tides, low tides form between the two high tides (Figure below). As the Earth rotates beneath the Moon, a single spot will experience two high tides and two low tides every day.

[Figure 5]

The gravitational attraction of the Moon to ocean water creates the high and low tide

The tidal range is the difference between the ocean level at high tide and the ocean at low tide (Figure below). The tidal range in a location depends on a number of factors, including the slope of the seafloor. Water appears to move a greater distance on a gentle slope than on a steep slope.

[Figure 6]

The tidal range is the difference between the ocean level at high tide and low tide.

Monthly Tide Patterns

If you look at the diagram of high and low tides in the Figure above, you’ll see that tides are waves. So when the Sun and Moon are aligned, what do you expect the tides to look like?

Waves are additive so when the gravitational pull of both bodies is in the same direction the high tides add and the low tides add (Figure below). Highs are higher and lows are lower than at other times through the month. These more extreme tides, with a greater tidal range, are called spring tides. Spring tides don't just occur in the spring; they occur whenever the Moon is in a new-moon or full-moon phase, about every 14 days.

[Figure 7]

Spring tides occur when the tidal bulges from the Moon and Sun are aligned. The Moon is full in this image; in the bottom image the Moon would appear as a new moon.

Neap tides are tides that have the smallest tidal range, and they occur when the Earth, the Moon, and the Sun form a 90^o angle (Figure below). They occur exactly halfway between the spring tides, when the Moon is at first or last quarter. How do the tides add up to create neap tides? The Moon's high tide occurs in the same place as the Sun's low tide and the Moon's low tide in the same place as the Sun's high tide. At neap tides, the tidal range relatively small.

[Figure 8]

Neap tides occur when the Earth, the Sun, and the Moon form a right angle; the Moon is in its first or third quarter.

High tides occur about twice a day, about every 12 hours and 25 minutes. The reason is that the Moon takes 24 hours and 50 minutes to rotate once around the Earth so the Moon is over the same location 24 hours and 50 minutes later. Since high tides occur twice a day, one arrives each 12 hours and 25 minutes. What is the time between a high tide and the next low tide?

Some coastal areas do not follow this pattern at all. These coastal areas may have one high and one low tide per day or a different amount of time between two high tides. These differences are often because of local conditions, such as the shape of the coastline that the tide is entering.

Surface Currents

Ocean water moves in predictable ways along the ocean surface. Surface currents can flow for thousands of kilometers and can reach depths of hundreds of meters. These surface currents do not depend on weather; they remain unchanged even in large storms because they depend on factors that do not change.

Surface currents are created by three things:

global wind patterns

the rotation of the Earth

the shape of the ocean basins

Surface currents are extremely important because they distribute heat around the planet and are a major factor influencing climate around the globe.

Global Wind Patterns

Winds on Earth are either global or local. Global winds blow in the same directions all the time and are related to the unequal heating of Earth by the Sun -- that is, more solar radiation strikes the equator than the polar regions –- and the rotation of the Earth -- that is, the Coriolis effect. The causes of the global wind patterns will be described in detail in the Earth's Atmosphere chapter.

Water in the surface currents is pushed in the direction of the major wind belts:

trade winds: east to west between the equator and 30^oN and 30^oS

westerlies: west to east in the middle latitudes

polar easterlies: east to west between 50^o and 60^o north and south of the equator and the north and south pole

Earth’s Rotation

Wind is not the only factor that affects ocean currents. The Coriolis effect describes how Earth’s rotation steers winds and surface ocean currents (Figure below). Coriolis causes freely moving objects to appear to move to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The objects themselves are actually moving straight, but the Earth is rotating beneath them, so they seem to bend or curve.

An example might make the Coriolis effect easier to visualize. If an airplane flies 500 miles due north, it will not arrive at the city that was due north of it when it began its journey. Over the time it takes for the airplane to fly 500 miles, that city moved, along with the Earth it sits on. The airplane will therefore arrive at a city to the west of the original city (in the Northern Hemisphere), unless the pilot has compensated for the change. So to reach his intended destination, the pilot must also veer right while flying north.

As wind or an ocean current moves, the Earth spins underneath it. As a result, an object moving north or south along the Earth will appear to move in a curve, instead of in a straight line. Wind or water that travels toward the poles from the equator is deflected to the east, while wind or water that travels toward the equator from the poles gets bent to the west. The Coriolis effect bends the direction of surface currents to the right in the Northern Hemisphere and left in the Southern Hemisphere.

[Figure 9]

The Coriolis effect causes winds and currents to form circular patterns. The direction that they spin depends on the hemisphere that they are in.

Coriolis effect is demonstrated using a metal ball and a rotating plate in this video. The ball moves in a circular path just like a freely moving particle of gas or liquid moves on the rotating Earth (5b)

 

 

 

 

Shape of the Ocean Basins

When a surface current collides with land, the current must change direction. In the Figure below, the Atlantic South Equatorial Current travels westward along the equator until it reaches South America. At Brazil, some of it goes north and some goes south. Because of Coriolis effect, the water goes right in the Northern Hemisphere and left in the Southern Hemisphere.

[Figure 10]

The major surface ocean currents.

You can see on the map of the major surface ocean currents that the surface ocean currents create loops called gyres(Figure below). The Antarctic Circumpolar Current is unique because it travels uninhibited around the globe. Why is it the only current to go all the way around?

[Figure 11]

The ocean gyres. Why do the Northern Hemisphere gyres rotate clockwise and the Southern Hemisphere gyres rotate counterclockwise?

 

 

 

 

Local Surface Currents

The surface currents described above are all large and unchanging. Local surface currents are also found along shorelines (Figure below). Two are longshore currents and rip currents.

[Figure 12]

Longshore currents move water and sediment parallel to the shore in the direction of the prevailing local winds.

Rip currents are potentially dangerous currents that carry large amounts of water offshore quickly. Each summer in the United States at least a few people die when they are caught in rip currents.

Effect on Global Climate

Surface currents play an enormous role in Earth’s climate. Even though the equator and poles have very different climates, these regions would have more extremely different climates if ocean currents did not transfer heat from the equatorial regions to the higher latitudes.

The Gulf Stream is a river of warm water in the Atlantic Ocean, about 160 kilometers wide and about a kilometer deep. Water that enters the Gulf Stream is heated as it travels along the equator. The warm water then flows up the east coast of North America and across the Atlantic Ocean to Europe (Figure below). The energy the Gulf Stream transfers is enormous: more than 100 times the world's energy demand.

The Gulf Stream's warm waters raise temperatures in the North Sea, which raises the air temperatures over land between 3 to 6^oC (5 to 11^oF). London, U.K., for example, is at the same latitude as Quebec, Canada. However, London’s average January temperature is 3.8^oC (38^oF), while Quebec’s is only -12^oC (10^oF). Because air traveling over the warm water in the Gulf Stream picks up a lot of water, London gets a lot of rain. In contrast, Quebec is much drier and receives its precipitation as snow.

[Figure 13]

In a satellite image of water temperature in the western Atlantic it is easy to pick out the Gulf Stream, which brings warmer waters from the equator up eastern North America.

Deep Currents

Thermohaline circulation drives deep ocean circulation. Thermo means heat and haline refers to salinity. Differences in temperature and in salinity change the density of seawater. So thermohaline circulation is the result of density differences in water masses because of their different temperature and salinity.

What is the temperature and salinity of very dense water? Lower temperature and higher salinity yield the densest water. When a volume of water is cooled, the molecules move less vigorously so same number of molecules takes up less space and the water is denser. If salt is added to a volume of water, there are more molecules in the same volume so the water is denser.

Changes in temperature and salinity of seawater take place at the surface. Water becomes dense near the poles. Cold polar air cools the water and lowers its temperature, increasing its salinity. Fresh water freezes out of seawater to become sea ice, which also increases the salinity of the remaining water. This very cold, very saline water is very dense and sinks. This sinking is called downwelling.

This video lecture discusses the vertical distribution of life in the oceans. Seawater density creates currents, which provide different habitats for different creatures (5d):

 

 

 

 

Two things then happen. The dense water pushes deeper water out of its way and that water moves along the bottom of the ocean. This deep water mixes with less dense water as it flows. Surface currents move water into the space vacated at the surface where the dense water sank (Figure below). Water also sinks into the deep ocean off of Antarctica.

[Figure 14]

Cold water (blue lines) sinks in the North Atlantic, flows along the bottom of the ocean and upwells in the Pacific or Indian. The water then travels in surface currents (red lines) back to the North Atlantic. Deep water also forms off of Antarctica.

Since unlimited amounts of water cannot sink to the bottom of the ocean, water must rise from the deep ocean to the surface somewhere. This process is called upwelling (Figure below).

[Figure 15]

Upwelling forces denser water from below to take the place of less dense water at the surface that is pushed away by the wind.

Generally, upwelling occurs along the coast when wind blows water strongly away from the shore. This leaves a void that is filled by deep water that rises to the surface.

Upwelling is extremely important where it occurs. During its time on the bottom, the cold deep water has collected nutrients that have fallen down through the water column. Upwelling brings those nutrients to the surface. Those nutrient support the growth of plankton and form the base of a rich ecosystem. California, South America, South Africa, and the Arabian Sea all benefit from offshore upwellin

Upwelling also takes place along the equator between the North and South Equatorial Currents. Winds blow the surface water north and south of the equator so deep water undergoes upwelling. The nutrients rise to the surface and support a great deal of life in the equatorial oceans.

Lesson Summary

Ocean waves are energy traveling through the water.

Most ocean waves are generated by wind. Tsunami are exceptionally long wavelength waves usually caused by earthquakes.

Tides are produced by the gravitational pull of the Moon and Sun.

Spring tides have large tidal ranges and occur at full and new moons, when Earth, Moon, and Sun are all aligned.

Neap tides have low tidal ranges and occur at first and last quarter moons, when the Moon is at right angles to the Sun.

Ocean surface currents are produced by global winds, the Coriolis effect and the shape of each ocean basin.

The Pacific and Atlantic Oceans have a circular pattern of surface currents called gyres that circle clockwise in the Northern Hemisphere and counterclockwise in the Southern. The Indian Ocean only has a counterclockwise gyre.

Surface ocean circulation brings warm equatorial waters towards the poles and cooler polar water towards the equator.

Thermohaline circulation drives deep ocean currents.

Upwelling of cold, nutrient-rich waters creates biologically rich areas where surface waters are blown away from a shore, or where equatorial waters are blow outward.